1. Field of the Invention
The present invention relates generally to a process for the recovery of n-butenes from methyl tertiary butyl ether (MTBE) made by the reaction of methanol with isobutene contained within a mixed C.sub.4 stream containing isobutene, normal butene and butanes. More particularly the invention relates to a catalytic distillation process which includes an oxygenate removal unit wherein light ends and light oxygenated compounds are separated from unreacted C.sub.4 's utilizing a distillation column. Most particularly, the invention relates to the use of a catalyst in the lower end of the oxygenate recovery column to convert heavier oxygenated compounds, such as MTBE and tertiary butyl alcohol (TBA) back to their constituent components for ease of separation.
2. Related Art
The production of ethers by the reaction of an isoolefin and an alcohol is a well known, e.g., U.S. Pat. Nos. 3,121,124; 3,270,081; 3,170,000; 3,629,478 and 3,634,534. One commercial operation utilizes the simultaneous reaction and distillation of the products from the reactants. This procedure has become known as catalytic distillation and has been adapted to MTBE production as disclosed in U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687 and 4,918,243. In the catalytic distillation process the catalyst is part of the distillation system acting as both catalyst and distillation structure. In addition to the above cited patents specific catalytic distillation structures are disclosed in commonly assigned U.S. Pat. Nos. 4,215,011; 4,242,530; 4,302,356; 4,443,559 and 4,482,775.
Briefly, a preferred and commercial catalyst structure described in the catalytic distillation process comprises a cloth belt with a plurality of pockets spaced along the belt and containing particulate catalyst material, said cloth belt being wound in a helix about a spacing material such as stainless steel knitted mesh. These units are then disposed in the distillation column reactor. In addition, commonly assigned U.S. Pat. Nos. 4,443,559 and 4,250,052 disclose a variety of catalyst structures for this use and are incorporated herein.
Since the feed in MTBE production is generally a mix C.sub.4 stream, there is usually a C.sub.4 stream, e.g. n-butenes separated and purified for other use.
The separation of the reactants from the ether products is fairly efficient. However, the unreacted methanol and C.sub.4 's which are taken overhead must then be separated. This presents somewhat of a problem because methanol forms various azeotropes in the C.sub.4 stream depending upon the pressure. Dimethyl ether (DME) is also a usual by-product.
In acid alkylations one problem caused by the presence of dimethyl ether (DME) and other oxygenated compounds is the use of more acid than the alkylation per se. In the case of sulfuric acid alkylations this may be a tolerable detriment, however, in the case of HF alkylations an acid soluble oil is formed which interferes with the separation of the alkylate product from the acid and may lead to fouling of the alkylation unit.
One common way to separate the methanol from the C.sub.4 stream utilizes distillation and water washing the mixture of C.sub.4 and methanol to remove the methanol. The water washing step leaves residual amounts of methanol (up to 2 wt %.), MTBE (50 ppm) and DME (500 ppm). The water wash could remove the DME but would require excessive amounts of wash water. This final water washed product is distilled to remove the DME and methanol.
One method of removing all oxygenated compounds is to subject the mixture to an adsorbent bed of molecular sieve or silica gel. See for example U.S. Pat. No. 4,814,517. Additionally, U.S. Pat. Nos. 4,575,566 and 4,575,567 disclose the use and regeneration of such adsorbents.
Another method is disclosed in and commonly assigned U.S. Pat. No. 5,122,236. The process disclosed therein utilizes the distillation column prior to the water wash step, but takes up to 40% overheads which insures that essentially all of the methanol and DME are taken overhead. The overheads are then condensed and subjected to the water wash which requires less water because of the smaller volume. The hydrocarbons in the condensed overheads are separated from the water phase containing the DME and methanol and returned to the distillation column.
However, to utilize either of the distillation techniques it is necessary to insure that there are no heavy oxygenates, such as MTBE or TBA (another by product of the MTBE process) present in the feed to the column as they would simply be taken out with the bottoms in the C.sub.4 product stream.